1. Field of the Invention
The present invention relates in general to the field of electronics and lighting, and more specifically to a system and method for varying colors in a dimmable lighting device using stable color temperature light sources.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. Gas discharge light sources, such as fluorescent, mercury vapor, low pressure sodium, and high pressure sodium lights and electroluminescent light sources, such as a light emitting diode (LED), represent two categories of light source alternatives to incandescent lights. LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives such as the reduction of mercury.
Incandescent lights generate light by passing current through a filament located within a vacuum chamber. The current causes the filament to heat and produce light. The filament produces more heat as more current passes through the filament. For a clear vacuum chamber, the temperature of the filament determines the color of the light. A lower temperature results in yellowish tinted light and a high temperature results in a bluer, whiter light.
Gas discharge lamps include a housing that encloses gas. The housing is terminated by two electrodes. The electrodes are charged to create a voltage difference between the electrodes. The charged electrodes heat and cause the enclosed gas to ionize. The ionized gas produces light. Fluorescent lights contain mercury vapor that produces ultraviolet light. The housing interior of the fluorescent lights include a phosphor coating to convert the ultraviolet light into visible light.
LEDs are semiconductor devices and are driven by direct current. The lumen output intensity (i.e. brightness) of the LED varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED, and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the white LEDs or by reducing the average current through pulse width modulation.
The color characteristic of light is the measure of the distribution of power over the visible spectrum. The visible spectrum has a wavelength range of approximately 400 nanometers (violet) to 700 nanometers (red). The color characteristic of light is commonly defined in terms of color temperature. Thus, although the light emitted by a light source has energy spread among multiple frequencies, the light is perceived to have a particular color that can be defined in terms of a particular color temperature. Table 1 depicts an exemplary correlation between a particular light source and the color temperature of the light source.
TABLE 1Light SourceColor temperature (Kelvin)Skylight (bluesky)12,000-20,000Average summer shade8000Light summer shade7100Typical summer light (sun + sky)6500Daylight fluorescent6300Xenon short-arc6400Overcast sky6000Clear mercury lamp5900Sunlight (noon, summer, mid-latitudes)5400Design white fluorescent5200Special fluorescents used for color evaluation5000Daylight photo flood4800-5000Sunlight (early morning and late afternoon)4300Brite White Deluxe Mercury lamp4000Sunlight (1 hour after dawn)3500Cool white fluorescent3400Photo flood3400Professional tungsten photographic lights3200100-watt tungsten halogen3000Deluxe Warm White fluorescent2950100-watt incandescent287040-watt incandescent2500High-pressure sodium light2100Sunlight (sunrise or sunset)2000Candle flame1850-1900Match flame1700
Dimming a light source saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. Many facilities, such as homes and buildings, include light source dimming circuits (referred to herein as a “dimmer”).
FIG. 1 depicts a lighting circuit 100 with a conventional dimmer 102 for dimming incandescent light source 104 in response to inputs to variable resistor 106. The dimmer 102, light source 104, and voltage source 108 are connected in series. Voltage source 108 supplies alternating current at line voltage Vline. The line voltage Vline can vary depending upon geographic location. The line voltage Vline is typically 110-120 Vac or 220-240 Vac with a typical frequency of 60 Hz or 70 Hz. Instead of diverting energy from the light source 104 into a resistor, dimmer 102 switches the light source 104 off and on many times every second to reduce the total amount of energy provided to light source 104. A user can select the resistance of variable resistor 106 and, thus, adjust the charge time of capacitor 110. A second, fixed resistor 112 provides a minimum resistance when the variable resistor 106 is set to 0 ohms. When capacitor 110 charges to a voltage greater than a trigger voltage of diac 114, the diac 114 conducts and the gate of triac 116 charges. The resulting voltage at the gate of triac 116 and across bias resistor 118 causes the triac 116 to conduct. When the current I passes through zero, the triac 116 becomes nonconductive, i.e. turns ‘off’). When the triac 116 is nonconductive, the dimmer output voltage VDIM is 0V. When triac 116 conducts, the dimmer output voltage VDIM equals the line voltage Vline. The charge time of capacitor 110 required to charge capacitor 110 to a voltage sufficient to trigger diac 114 depends upon the value of current I. The value of current I depends upon the resistance of variable resistor 106 and resistor 112. Thus, adjusting the resistance of variable resistor 106 adjusts the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM is equivalent to adjusting the phase angle of dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM adjusts the average power to light source 104, which adjusts the intensity of light source 104.
FIG. 2 depicts a spectral power distribution graph 200 representing changes in spectral power distribution over the visible spectrum for a white LED, green LED, and incandescent light sources for high and low drive currents. A light source is dimmed by decreasing the drive current supplied to the light source. Dimming an incandescent light source results in a dramatic change of spectral power distribution and, thus, results in a dramatic change in color temperature. For example, dimming a 100W incandescent light bulb by 75% of full intensity results in a color change from bluish-white to shade of yellow, such as amber. Reducing the current to an LED, such as a green and white LED, reduces the intensity of the LED, but the spectral power distribution remains essentially the same. Thus, the color temperature of an LED changes very little. Gas discharge lights exhibit a behavior very similar to LEDs for various dimming levels.
FIG. 3 depicts a graphical relationship 300 between dimming levels and color temperatures for a non-incandescent light source. The color temperature of a lighting device having non-incandescent light sources can be changed by varying a mix of non-incandescent light sources. However, regardless of the mix of non-incandescent light sources in a lighting device, varying the dimming level to the lighting device changes the intensity of the light sources not the color temperature of the lighting device.
Although lighting devices having one or more non-incandescent light sources can be dimmed, dimming non-incandescent light sources does not result in familiar color temperature changes associated with incandescent light sources.